PROJECT SUMMARY/ABSTRACT Failure to respond to seasonal change has severe consequences for an organism?s ability to survive and reproduce. For humans, seasonal oscillation in the surrounding environment, especially the amount of light, can cause Seasonal Affective Disorder, as well as cardiovascular and immunity-related diseases. In animals and plants, reproduction is precisely aligned with specific seasons. Many organisms, including humans, have evolved sensing mechanisms to prepare for upcoming seasonal changes by adjusting homeostasis, physiology and development. The long-term goal of our research program is to elucidate the molecular mechanisms by which organisms measure seasonal changes, particularity in day length and temperature. Although we know that the interplay between external stimuli (light and temperature) and the internal circadian clock orchestrates seasonal responses, the molecular regulatory networks involved have remained largely elusive. Seasonal time measurement has been one of the important topics in chronobiology for decades, and we have learned that similar types or structures of these networks exist in both animals and plants. Among the model organisms used in this research, our knowledge of the model plant Arabidopsis is the most advanced. In Arabidopsis, ambient light and temperature differences are processed through the molecular clock network to regulate the transcription of a florigen (flower-inducing) gene called FLOWERING LOCUS T (FT). Our major focus is elucidating how FT transcription is regulated. Although the circadian clock-dependent seasonal sensing mechanism is the major controller of FT expression, other external and internal information is channeled into the regulation of FT transcription to precisely determine the timing of flowering. In Aim 1 of this proposal, we will obtain more precise epigenomic and transcriptomic information in both FT-expressing cells and cells that do not express FT at the tissue/cell-type levels. With this information, we will be able to organize our current understanding of FT regulation at the whole plant level into more precise tissue/cell-type specific regulation. Through our study of plants grown in nature, we recently found an FT transcription regulation controlled by light and the circadian clock that had been completely uncharacterized up to this point. We will study the molecular mechanisms underlying this regulation in Aim 2. Our work in seasonal sensing mechanisms has provided functional knowledge about the photoreceptor FKF1. In Aim 3, we will obtain more precise knowledge about how this photoreceptor is built by tuning the length of light-excited states and analyzing changes in biochemical output function. This information will help us understand how the photochemical features of this photoreceptor control functional outputs; it will also likely provide us with more optogenetic tools. The findings will have a large impact on plant research and our broader understanding of seasonal sensing and circadian clocks in mammals and other systems, including humans.